Ultrasonic vibratory probes have been used in automated chemical analyzers where a probe is used to automatically withdraw liquid from a vessel containing liquid samples or reagents. Robotic probes of other types have been similarly used in such instruments.
In automated analyzer systems, it is desirable for the probe element to be able to detect the surface of the liquid so the probe's contact with the vessel's contents is minimized, thus, decreasing the possibility of carry over of liquid on the probe.
Various types of level sensors have been developed to provide such surface or level detection. One type of level sensor used with robotic probes in chemical analyzers are referred to as capacitive level sensors. They operate on the principle that every conductor exhibits a finite electrical capacitance. When a probe having a capacitive level sensor touches a liquid, the higher dielectric constant and greater surface area of the liquid results in an increased probe capacitance that can be detected.
Among prior art capacitive liquid level sensors is the sensor described in Davis U.S. Pat. No. 4,977,786. The sensor disclosed therein is useful in a liquid pipetting system that includes an oscillator coupled to a pipette probe for applying a high frequency signal to the probe, the amplitude or phase of the oscillator being affected by the capacitance of the probe and comparator means that generates a level sensor signal according to the amplitude or phase of the oscillator to signal when the probe reaches the liquid surface. The probe also includes means used to vary the frequency of the oscillator in a repetitive manner.
It is desirable in automatic chemical analyzers systems to use an ultrasonically activated pipetting probe that may perform several functions including reagent mixing and level sensing. It is therefor desirable for the ultrasonic probe to also be able to detect the fluid surface in vessels containing reagents and sample.